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Understanding sea water key to climate change

“The changes are important because
variations of salinity and temperature are responsible for driving
deep ocean currents and the major vertical overturning circulations
of the world’s oceans,” Dr McDougall says.
“Getting these circulations right is central to the task of
quantifying the ocean’s role in climate change.”

“We feel we are sufficiently
well-advanced with our arguments to now go out to the oceanographic
community and propose adoption of new and more accurate
oceanographic variables that we suggest be called ‘Absolute
Salinity’, and ‘Conservative Temperature’.”
These more accurate variables will take the place of today’s
Practical Salinity and Potential Temperature.

Dr McDougall said the new definition
allows, for the first time, an accurate measure of the heat content
of seawater for inclusion in ocean models and climate
projections.

“To date, ocean models assume that
the heat content of seawater is proportional to a particular
temperature variable called “potential temperature”.
The new description of seawater allows us to measure the errors
involved by using this approximation while presenting a much more
accurate measure of the heat content of seawater, namely
Conservative Temperature. The difference is mostly less than
1º C at the sea surface, but it is important to correct for
these biases in ocean models.”

Sea water is a mixture of 96.5 per cent
pure water and 3.5 per cent other material, such as salts,
dissolved gases, organic substances, and undissolved particles.
Salinity, comprising the salts washed from rocks, is measured using
the conductivity of seawater. This technique assumes that the
composition of seawater is the same in all the world’s
oceans. It has been known for some time that there are small
variations in the composition of seawater around the globe, and the
SCOR working group is now in a position to recommend a practical
method for taking these variations into account. The changes in
salinity, while small, are a factor of about ten larger than the
accuracy with which scientists can measure salinity at sea.